kolibri: AttributeError: '_RSAPrivateKey' object has no attribute 'sign' (Morango)

Observed behavior

Just built 0.7.0-beta5 and running it from the Debian, the setup process fails after completing the final step:

Expected behavior

Correctly create a key and quit.

User-facing consequences

Release blocker - or at least getting it explained is 😃

Errors and logs

INFO 2017-12-04 17:16:34,287 cli Going to daemon mode, logging to /var/kolibri/.kolibri/server.log
ERROR 2017-12-04 17:18:26,979 base Internal Server Error: /api/deviceprovision/
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/kolibri/dist/django/core/handlers/base.py", line 149, in get_response
    response = self.process_exception_by_middleware(e, request)
  File "/usr/lib/python3/dist-packages/kolibri/dist/django/core/handlers/base.py", line 147, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python3/dist-packages/kolibri/dist/django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/kolibri/dist/rest_framework/viewsets.py", line 87, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/kolibri/dist/rest_framework/views.py", line 466, in dispatch
    response = self.handle_exception(exc)
  File "/usr/lib/python3/dist-packages/kolibri/dist/rest_framework/views.py", line 463, in dispatch
    response = handler(request, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/kolibri/core/device/api.py", line 31, in create
    data = serializer.save()
  File "/usr/lib/python3/dist-packages/kolibri/dist/rest_framework/serializers.py", line 191, in save
    self.instance = self.create(validated_data)
  File "/usr/lib/python3/dist-packages/kolibri/core/device/serializers.py", line 57, in create
    facility = Facility.objects.create(**validated_data.pop('facility'))
  File "/usr/lib/python3/dist-packages/kolibri/dist/django/db/models/manager.py", line 122, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/kolibri/dist/django/db/models/query.py", line 401, in create
    obj.save(force_insert=True, using=self.db)
  File "/usr/lib/python3/dist-packages/kolibri/auth/models.py", line 969, in save
    super(Facility, self).save(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/kolibri/auth/models.py", line 706, in save
    super(Collection, self).save(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/kolibri/dist/mptt/models.py", line 1001, in save
    super(MPTTModel, self).save(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/kolibri/auth/models.py", line 163, in save
    self.ensure_dataset()
  File "/usr/lib/python3/dist-packages/kolibri/auth/models.py", line 975, in ensure_dataset
    super(Facility, self).ensure_dataset(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/kolibri/auth/models.py", line 175, in ensure_dataset
    inferred_dataset = self.infer_dataset(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/kolibri/auth/models.py", line 980, in infer_dataset
    self.dataset = FacilityDataset.objects.create()
  File "/usr/lib/python3/dist-packages/kolibri/dist/django/db/models/manager.py", line 122, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/kolibri/dist/django/db/models/query.py", line 401, in create
    obj.save(force_insert=True, using=self.db)
  File "/usr/lib/python3/dist-packages/kolibri/dist/morango/models.py", line 279, in save
    super(SyncableModel, self).save(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/kolibri/dist/morango/utils/uuids.py", line 117, in save
    self.id = self.calculate_uuid()
  File "/usr/lib/python3/dist-packages/kolibri/dist/morango/models.py", line 324, in calculate_uuid
    self._morango_source_id = self.calculate_source_id()
  File "/usr/lib/python3/dist-packages/kolibri/auth/models.py", line 113, in calculate_source_id
    self._morango_source_id = Certificate.generate_root_certificate("full-facility").id
  File "/usr/lib/python3/dist-packages/kolibri/dist/morango/certificates.py", line 81, in generate_root_certificate
    cert.sign_certificate(cert)
  File "/usr/lib/python3/dist-packages/kolibri/dist/morango/certificates.py", line 120, in sign_certificate
    cert_to_sign.signature = self.sign(cert_to_sign.serialized)
  File "/usr/lib/python3/dist-packages/kolibri/dist/morango/certificates.py", line 150, in sign
    return self.private_key.sign(value)
  File "/usr/lib/python3/dist-packages/kolibri/dist/morango/crypto.py", line 55, in sign
    signature = self._sign(message)
  File "/usr/lib/python3/dist-packages/kolibri/dist/morango/crypto.py", line 244, in _sign
    return self._private_key.sign(message, crypto_padding.PKCS1v15(), crypto_hashes.SHA256())
AttributeError: '_RSAPrivateKey' object has no attribute 'sign'

kolibri shell
INFO     Running Kolibri with the following settings: kolibri.deployment.default.settings.base
Python 3.5.2 (default, Nov 23 2017, 16:37:01) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> import morango
>>> morango.__version__
'0.0.2'
>>> try:
...     from M2Crypto import RSA as M2RSA
...     from M2Crypto import BIO as M2BIO
...     M2CRYPTO_EXISTS = True
... except:
...     M2CRYPTO_EXISTS = False
... 
>>> M2CRYPTO_EXISTS
False
>>> import cryptography
>>> cryptography.__file__
'/usr/lib/python3/dist-packages/cryptography/__init__.py'
>>> import cryptography
>>> cryptography.__version__
'1.2.3'

The C extensions are built…

$ ls /usr/lib/python3/dist-packages/kolibri/dist/cext/cp35/linux-x86_64/
asn1crypto  cffi  _cffi_backend.so  cryptography  idna	pycparser  pycparser-2.18.tar.gz  six.py

Steps to reproduce

I can release/upload the .deb installer with the error if necessary…

Context

• Kolibri version: 0.7.0-beta5
• Operating system: Ubuntu 16.04
• Browser: ?